Polymer polyols within the scope of the invention are products which can be obtained by the polymerization of ethylenically unsaturated compounds in the presence of polyether polyols (so-called "base polyols"), a macromer, and an enol ether of a specified formula. These can be used, for example, for the manufacture of polyurethane flexible foam materials. The principally used ethylenically unsaturated compounds are the monomers styrene and acrylonitrile, which are, for example, radically polymerized in polyether polyols as base polyols.
The production of such polymer polyols is, for example, described in the U.S. Pat. Nos. 3,383,351, 3,304,273, and Reissue 33,291, or in German Auslegeschrifien 1,152,536 and 1,152,537.
The polymer polyols in the ideal case are relatively low viscosity, finely dispersed, non-sedimenting dispersions of the polymerizate (preferably of an acrylonitrile/styrene-graft-copolymer)in the essentially unchanged polyether polyol. Characteristic identifiers for quality and applicability of the polymer polyols are viscosity, storage stability (sedimentation resistance) and small particle size. These characteristics are primarily influenced by the type and quantity ratio of the base materials. The solids content (monomer portion in the preparation) and the monomer ratio (for example, the styrene/acrylonitrile ratio) have an especially large influence on the quality of the final product.
The most important goals during the production of polymer polyols are achieving a high solids content (at least 40%) at the highest possible styrene percentage, and the lowest possible viscosity, with excellent product stability at the same time.
In order to achieve product stability, i.e the elimination of the formation of undesired, agglomerated polymer particles which precipitate from the continuous phase of the base polyol, these polymer particles must be stabilized during the polymer polyol production. This stabilization is effected on one hand through the building-in of a part of the molecules of the base polyether into the in-situ formed polymer. The effectiveness of stabilization is thereby favored on the other hand through a high molecular weight of the base polyethers, and through a high acrylonitrile portion in the monomer mixture. A nigh acrylonitrile content increases the color of the polymeric polyols, and correspondingly, the discoloration tendency of the flexible foam manufactured from them, and accordingly is undesirable. Another problem is that the viscosity of the polymer polyols is increased through the use of base polyols with higher molecular weights.
One possibility to obtain low viscosity polymer polyols with higher solids contents essentially independent from the molecular weight of the base polyol and from the acrylonitrile portion in the monomer mixture, is through the use of chain transfer agents. Some of the chain transfer agents commonly used in this polymerization technology include compounds such as, for example the usual mercaptans, halogenated alkanes, or alcohols.
Also known, is the use of enol ethers as chain transfer agents as described in EP-A 008,444, through which the production of polymer polyols having a relatively low viscosity at a relatively high solids content and styrene portion are made possible.
Another possibility for the stabilization of polymer polyols is the use of compounds that are compatible with the polyol phase, which contain ethylenically unsaturated, polymerizable groups. These so-called macromers copolymerize with the vinyl monomers such that the developing polymer particles become sterically stabilized by the polyether side chains, and are in this way protected against agglomeration and sedimentation.
The production of polymer polyols using macromers is disclosed in, for example, U.S. Pat. Nos. 3,652,639, 3,823,201, 4,460,715, 4,390,645, 5,093,412, and 4,342,840. The ethylenically unsaturated double bonds are introduced into the polyether polyols through reaction with cyclic, unsaturated carbonic acid anhydrides, such as maleic acid anhydride, and subsequent reaction with ethylene or propylene oxide; through esterification with acrylic or methacrylic acid (or derivatives thereof); through reaction with allylglycidyl ether; through reaction with an unsaturated isocyanate such as, for example, isocyanatoalkyl acrylate and isocyanatomethacrylate, 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)-benzene or NCO-functional adducts from a polyisocyanate and hydroxyethyl acrylate or hydroxypropyl acrylate.
Also, the combination of macromers with chain transfer agents such as mercaptans or alcohols, is described in German Auslegeschriff 2,500,274 E:P-A 0,190,769, as well as in EP-A 0,091,036.
Although the mentioned processes lead to polymer polyols which can basically be used as the polyol component for the manufacture of polyurethane flexible foams, further improvements are desirable. Also, the viscosities of higher solids content polymer polyols, especially those containing a higher styrene portion in the monomer mixture are still very high as can be seen from the literature, and often smaller agglomerized polymer particles can be observed in the residue, i.e. a finely dispersed product is not yet achieved.
Surprisingly, it was found that agglomerate-free polymer polyols with high solids content and low viscosities can be prepared when the polymer polyol is produced in the presence of a macromer of the type described above, and an enol ether of a specific formula as chain transfer agents.
Through the use of a combination of macromers with the special chain transfer agents of the enol ether type as demonstrated by the working examples herein, stable, finely dispersed polymer polyols are produced which have lower viscosities at a specific solids content and styrene portion, or contain a higher styrene content and/or solids content at a specific viscosity than previous products produced using other known combinations of chain transfer agents and macromers.